Field of the Invention
The present invention relates to telecommunication systems. More particularly, the present invention relates to a transmission architecture which make use of multiple antenna and amplifiers to transmit large symbols' constellations, that is compatible with low-cost, highly-efficient, nonlinear amplifiers, while allowing information directivity in the transmitted symbols.
Description of Related Art
Wireless communication systems must fulfill several requirements that include spectral and power efficiency, low interference level, robustness against jamming and security. In broadband communications requirements such as spectral efficiency and energy efficiency are equally important to support high transmission rates while minimizing the energy consumption of mobile devices. This can be achieved through the use of multilevel modulations and energy efficient amplification techniques.
Multilevel modulations can improve spectral efficiency of communication systems, despite the fact that this increase spectral efficiency may come at the expense of a reduced power efficiency, which is undesirable in systems where power consumption is a constraint, such as in broadband mobile wireless systems. Power efficiency can be assured through, an efficient amplification operation. However, as referred in [1], due to the envelope fluctuations of multilevel constellations, the amplifiers must work far from the saturation to avoid nonlinear effects.
The decomposition of a size-M multilevel modulations based on M-QAM (Quadrature Amplitude Modulation), Voronoi or M-PSK (Phase Shift Keying) constellations, into a sum of M′≤M BPSK (Binary-Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) or OQPSK (Offset QPSK) components with constant or quasi-constant envelope allows the use of nonlinear amplifiers, which can maximize the power efficiency of the transmission system. In the documents [2] and [3], it was proposed a representation method of multilevel constellations in terms of OQPSK components. However, the proposed representation was applied for the definition of coded schemes resulting from the combination of several nonlinear OQPSK components in which each coded schemes was decomposed. Moreover, a conventional amplification was employed for the resulting multilevel constellation. Thus, nonlinear distortion due to high power amplification was not avoided due to the envelope fluctuations of the resulting constellation.
Since QPSK signals can be viewed as a sum of two BPSK components in quadrature and OQPSK signal can be regarded as BPSK when referred to the appropriate carrier frequency, they both can be decomposed in BPSK sub-constellations.
One can describe a generic constellation as the sum of M′≤M BPSK or OQPSK sub-constellations, wherein the constellation symbols may be expressed as a function of the information bits β(m) in accordance with the expression
      a    n    =                    g        0            +                        g          1                ⁢                  b          n                      (            1            )                              +                        g          2                ⁢                  b          n                      (            2            )                              +                        g          3                ⁢                  b          n                      (            1            )                          ⁢                  b          n                      (            2            )                              +                        g          4                ⁢                  b          n                      (            3            )                              +      …        =                  ∑                  i          =          0                          M          -          1                    ⁢                        g          i                ⁢                              ∏                          m              =              1                        μ                    ⁢                                    (                              b                n                                  (                  m                  )                                            )                                      γ                              (                                  m                  ,                  i                                )                                                        for each αn∈A, where (γ(μ−1,i), γ(μ−2,i), . . . , γ(2,i),γ(1,i)) is the binary representation of i, bn(m)=2β(m)−1 denotes the polar representation of the bits, μ=log2(M) and gi is a complex coefficient associated to the definition of each sub-constellation BPSK, i.e. the corresponding amplitude and phase.
Since power-efficient constellations have zero mean, the complex coefficient associated to the zero order sub-constellation is null, i.e. g0=0 and we only need at maximum M′=M−1 BPSK signals to generate a multilevel constellation. In the particular case of M-QAM constellations it can be shown that all constellation symbols can be characterized with only M′=μ=log2(M) BPSK signals, since the remaining gi coefficients are zero. For instance, the 16-QAM with Gray mapping only needs four BPSK signals defined by the set of non-zero complex coefficients g6=j, g7=j, g8=2 and g9=1 (actually, this corresponds to only two QPSK constellations).
At the transmitter, the multilevel constellations are decomposed into a sum of BPSK or QPSK components which are amplified separately. This allows ally good power amplification efficiency together with the high spectral efficiency achieved by multilevel constellations.
To increase robustness of point-to-point communication systems against interference and malicious interception more highly directive beam of radiation can be used. Hence, by arranging elementary radiators into an array, a more directive beam of radiation can be obtained. This was shown in [4]. A more directive beam means that the antenna array will also have a higher gain. The other important requirement for a communication system is high ratio of the signal to interference. In the document [5] it is described the classical approach to achieve this which it is based on the suppression of interference and multipath signals and obtaining nulls in the directions of interfering signals. Current synthesis methods, as those described in [6] and [7], try to reduce interference by two ways: by reducing or even suppressing the side lobe level, whereas preserving the gain of the main beam; by introducing nulls in the radiation pattern in the directions where exists interference and jamming. However, for all these methods the transmitted signal by each antenna is the same in all directions although attenuated outside the main direction of the antenna, according to the array's radiation pattern.
In [8] it is disclosed a method of transmitting data based on an M-QAM modulation with nonlinear amplification. The transmission technique proposed in this application follows a different approach because the transmitter employs M′≤M antennas in parallel, on for each of the BPSK or QPSK signals in which the multilevel constellation is decomposed.
In [9] there are disclosed antenna arrays aimed to achieve a directive radiation pattern diagram, since the signals transmitted by the different antennas are correlated. The transmission structure proposed in this application, although based on one-dimensional or two-dimensional antenna arrays, does not achieve a directive radiation pattern since the signals transmitted by different antennas are uncorrelated, contrarily to what happens in [9].
Although it uses a set of antennas, the directivity of the disclosed method in the present application is introduced through a dependency on the configuration of constellations points on the desired direction of transmission. It should be noted that this directivity on the transmitted information is not accompanied by a maximization of radiation pattern in the desired transmission direction. Therefore, in contrast to the cases described in [9] cited document, the radiated power is not modified to maximize the radiated power in a given direction.
In the method disclosed in the present application, the signals in each antenna have different phases and data, since they use independent and uncorrelated bit streams. There is no a maximization of the radiation pattern on a specific direction but a constellation's configuration that depends on the desired transmission direction. Consequently, the optimization of the transmitted constellation in the desired direction does not change the power radiated in the same direction.
It should be also noted that the spatial factor associated with the transmitted constellation is not associated to the radiation pattern, contrarily to what was proposed in [9]. The signals at each of the antennas are independent, so it is not possible to define a spatial factor for the radiation pattern for the antenna array. In the present application the various constituent signals suffer phase rotations according to their position on the set of transmit antennas so that the constellation is optimized in the desired direction.
It should be mentioned that under these conditions and contrarily to the usual approach it is not possible to define a spatial factor for the antenna array that affects the radiated field. However, it can be defined an equivalent special factor that affects the constellation configuration and each sub-constellation. The closest case consists on a transmission of M′ signals in parallel, similarly to what happens in a MIMO (Multiple-Input Multiple-Output) system, but unlike the MIMO wherein each signal is associated to a well defined signal now each signal belongs to one of sub-constellations in which the constellation is decomposed. Also, unlike the MIMO without pre-coding, where at the receiver each signal can be received and decoded separately, the receiver for the proposed method needs to combine the M′ received signals to generate the transmitted symbol and only after this operation may decode the transmitted bits.
Document [10] discloses a transmission method to increase the system's throughput and where there is no decomposition of the constellation into sub-constellations. Moreover, the method uses a single antenna, since all signals after the multiplication by the spreading sequences are combined and transmitted by only one antenna.
In [11] there are disclosed methods for nonlinear encoding of 16-OQAM schemes, based on two nonlinear OQPSK signals specially designed to allow higher amplification efficiency due to its robustness against nonlinear distortions.
In [12] there are disclosed pragmatic FDE (Frequency Domain Equalization) receivers that have low complexity but allow excellent performance, even for large QAM constellations and highly non-uniform offset constellations. A more detailed study about the reason behind the poor performance of modulations equalized with conventional FDE schemes is also presented.
The decomposition of the constellations of the present application is generic since that the constituent signals may be of BPSK, QPSK or OQPSK when there is any temporal offset between in-phase and quadrature components, and is not restricted to the serial OQPSK format described by (Eq. 9) and (Eq. 10) in [11] and [12].
It should be stressed out that this format can be also associated with a representation of OQPSK signals based on Volterra decomposition that can be applied to describe the nonlinear effects of memoryless bandpass nonlinearity on OQPSK type signals. One advantage of this format is that it remains invariant after passing through a non-linearity, which makes possible the analytical characterization of non-linear effects. In the present method, such a restriction is not applied since the signals used in each of the branches have constant or almost constant envelope, by selecting a pulse shape like MSK (Minimum Shift Keying), GMSK (Gaussian MSK) or other pulse shapes assuring a very low level of envelope fluctuations unlike the usual square-root raised-cosine pulses. This aspect is omitted in the diagram of the transmitter, where the modulator may be a BPSK, QPSK or OQPSK modulator with changes in the pulse shapes.